1. Field of the Invention
The present invention relates to a wheel-head feed mechanism for a highly efficient grinder in which a grinding wheel is fed into a workpiece at high speed in order to efficiently grind the workpiece. The present invention also relates to a grinder that utilizes such a wheel-head feed mechanism.
2. Discussion of the Related Art
Recently, as in the field of cutting work, further improvement in productivity is demanded in the field of grinding work. In grinding work for cam shafts used in automobile engines, there has been employed a so-called plunge grinding scheme in which a grinding wheel is fed toward a cam on a cam shaft--which is rotated about the main spindle axis--in a radial direction thereof. In such plunge grinding, in order to increase the machining efficiency of the cam, attempts have been made to increase the speed of plunge infeed of the grinding wheel toward the cam.
In a conventional cam grinder of such a type, a wheel head is slidably guided by a pair of guides provided on a bed, and is advanced and retracted through use of a feed mechanism in which rotational drive force of a servomotor fixed to the bed is transmitted to the wheel head via a ball-screw mechanism.
More specifically, a nut unit of the ball-screw mechanism is disposed between a pair of guided surfaces of the wheel head, which are formed to correspond to the above-described pair of guide surfaces on the bed, and is fixed to the bottom surface of the wheel head via a bracket. Therefore, a wheel spindle rotatably supported at the upper front portion of the wheel head is quite distant from the axis of the ball-screw in screw-engagement with the nut unit.
Also, in order to eliminate backlash, the nut unit is composed of two nut members that are disposed in tandem with a spacer interposed therebetween.
Further, in the conventional cam grinder, an electric motor is disposed on the rear top surface of the wheel head, and the wheel spindle is driven via a pair of pulleys and belts.
In the wheel-head feed mechanism having the above-described structure, since the nut unit is constituted through tandem disposition of two nut members, the length of the nut unit becomes large, resulting in an inevitable increase in the length of the effective thread portion of the ball screw to be engaged with the nut unit. Consequently, the overall length of the ball-screw apparatus increases. Therefore, the ball-screw apparatus must be disposed under the wheel head parallel thereto in order to reduce the installation space of the cam grinder.
Also, since the overall length of the ball screw is large, when the ball screw is rotated at high speed, influence of rotational deflection of the ball screw becomes remarkable, resulting in deterioration in the feed accuracy of the wheel head, which in turn results in deterioration in the machining accuracy of a ground surface of a workpiece. In order to solve this problem, a deflection absorbing member such as a floating plate must be disposed between the nut unit and the wheel head, or other measures must be taken. Therefore, the conventional wheel-head feed mechanism has had a complicated structure.
Further, since the axis of the ball screw disposed under the wheel head is quite distant in the vertical direction from the axis of the wheel spindle disposed at the upper portion of the wheel head, a grinding resistance acting on the grinding point of the grinding wheel attached to one end of the wheel spindle produces a moment that deviates (or rotates) the wheel head about the engagement point between the nut unit and the ball screw, in a plane perpendicular to the wheel spindle. This moment induces pitching motion of the wheel head, thereby hindering smooth feed, which becomes a cause of deterioration in the machining accuracy of a cam surface. This problem is especially remarkable in a cam grinder in which the grinding point reciprocates while crossing a horizontal plane in which the workpiece spindle and the axis of the wheel spindle lie.
In order to enable high speed feed of the wheel head, it is desired to reduce the mass of the wheel head itself. However, since the wheel head of the conventional cam grinder must be made long in order to secure an installation space for an electric motor for driving the wheel spindle to be mounted on the rear portion of the wheel head, reduction in the size of the wheel head has been difficult.